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Swift Knowledge Database

Variables

private var x: Bool = false

let y: String = "Hello World"

States

@State private var showModal: Bool = false // For primitive data types

print(showModal) // -> false
print($showModal) // -> false (as Binding)
print(_showModal) // -> false (to initialize Value) Does not re-render when changed

@StateObject private var myObject: myObjectClass = myObjectClass() // for objects

Bindings

Bindings in Swift allow a child function or a child view to read and modify a variable defined in a parent view. By using a binding, changes made to the variable in the child view are reflected in the parent view.

Declaration

To indicate that a structure or class needs a binding to a variable, the @Binding property wrapper declaration is used. For example:

@Binding var isShown: Bool
@ObservedObject var myObject: myObjectClass

This declaration means that isShown is a binding to a Bool variable and myObject is a binding to myObjectClass which is passed from the parent view.

Usage

A binding is typically passed from a parent view to a child view. In the parent view, a @State variable is used to manage the state:

@State private var isShown: Bool = false
@StateObject private var myObject: myObjectClass = myObjectClass()

Then, a binding to this State variable is passed to the child view:

ChildView(isShown: $isShown)
ChildView(myObject: $myObject)

Here, $isShown is a binding to the @State variable isShown.

Important to Note

  • A source of truth has to be private!
  • A @Binding can only be bound to a @State variable or another source of truth (like @ObservedObject or @EnvironmentObject).
  • If a variable is declared as a @Binding, it expects a binding as input. Passing a regular value directly (without the $ prefix) will result in a compilation error.

Structs

  • Leightweight
  • Performant
  • Value Type

Classes

  • Inheritance
  • Reference Type

Visibility

  • public
  • internal
  • fileprivate
  • private

(Return-) Types

To describe the type of a variable, use :.

private var x: String = "Hello World!"

To describe the return type of a method or function, use ->.

private var foo: (String, Int) -> Void

public var bar: () -> Int

func foobar() -> Int {
    return 3
}